Acoustic-based imaging techniques are used in both diagnostic and therapeutic procedures. Contrast agents are often used with acoustic-based imaging techniques to enhance image quality. Contrast agents are able to increase acoustic backscatter, create an ultrasonic echo, and create resonance effects, resulting in high echogenicity. Materials with high echogenicity are better able to return a signal when exposed to an acoustic wave. The increased echogenic difference between the contrast agent and the surrounding material increases the signal contrast difference and is used to create higher resolution images.
Gas filled microbubbles are often used as contrast agents. The microbubbles are generally less than one millimeter in diameter. But, in many acoustic-based imaging applications, the microbubble diameter is less than 6 micrometers, which is smaller than the size of red blood cells and allows the microbubbles to freely flow through the entire circulatory system.
In some instances, simple air bubbles are used. The surface of the air bubble reflects the acoustic waves and the gas core of the bubble may compress in the acoustic field to generate an echo. Air bubbles, however, have relatively short useful lifespans because the water-soluble gases in air leak into the surrounding solution. To increase the useful life, bubbles may be filled with heavy gases that are less water-soluble.
Ultrasound contrast agents are approved by the United States Food and Drug Administration (FDA) for medical imaging. Presently, there are two such agents approved by the FDA in the United States. Both of these agents are based on perfluoropropane gas. One is coated by phospholipid; the other is coated by serum albumin. In Europe, clinical trials are under way with an agent based on phospholipid-coated perfluorobutane microbubbles.